22.55 “Principles of Radiation Interactions”
Overview of Cell Biology
The Cell
• Cells are the fundamental unit of life; the structural and functional unit of all
living organisms.
• The cell theory: All organisms are composed of cells and all cells come from
pre-existing cells.
• In single-celled organisms such as bacteria and protozoa, each cell is
independent.
• In multi-celled organisms, function is distributed among different specialized
cells.
• Cells to tissues to organs to organisms…
• Biological systems use cells to build higher levels of organization, but the cell
remains the fundamental unit. Radiation effects at the cellular level can affect
the higher-level organization.
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[Lodish, 2000]
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What can cells do?
Cells can:
• use DNA as hereditary material,
• use proteins as catalysts
• reproduce
• transform matter and energy
• respond to their environment
Sizes and shapes vary
0.5 µm bacteria to a several cm hen egg (yolk)
shapes can be spherical, flattened columnar, cuboidal.
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The cell membrane
• The interior of cells is an aqueous environment.
• Most of the intracellular molecules are water-soluble.
• Most of the chemical reactions carried out inside cells require an aqueous
environment.
• The environment around cells is also basically an aqueous one.
• Bodily fluids and blood are aqueous solutions of proteins and small
molecules.
• Chemically (pH, ionic strength) blood is similar to sea water.
For cells to maintain integrity they must be surrounded by a membrane through
which water cannot flow. A membrane composed of fatty acid molecules served
this purpose.
All cells have a cell membrane, a two-layered shell of phospholipids. All
biological membranes have the same basic phospholipid bilayer structure.
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Other functions of membranes
Membranes serve many functions other
than segregating inside from outside.
Many proteins are associated with
membranes, either embedded in the
membrane, or attached to the surface.
• Transport of ions or macromolecules
• Energy storage. Ion concentration gradients serve as source of potential
energy.
• Receptors for signal transduction
• Adherence. Cell-cell contacts.
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Two major divisions of cells.
Prokaryotes (before nucleus). Lack a defined nucleus. Have a simplified internal
organization. Have only a plasma membrane and a nucleoid, no internal
membranes.
All are single celled organisms (e.g., bacteria, blue-green algae)
Eukaryotes (typical or true nucleus). All members of the plant and animal
kingdoms are eukaryotes. Eukaryotes have a more complicated internal structure
including a defined, membrane-limited nucleus, and several distinct, membranedelimited organelles.
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[Lodish, 2000]
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Two major divisions within the cell.
Nuclear region (information storage and processing)
Cytoplasm (diverse functions, organelles)
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[from Lodish, 2000]
Some definitions:
Plasma membrane: surrounds the outside of the cell, 7-8 nm thick, responsible
for transport, receptors (signal transduction) adherence, immune recognition.
Nuclear envelope: separates the nucleus and the cytoplasm; a double membrane
with nuclear pores (channels for conducting materials between the nucleus and the
cytoplasm)
Nucleus. Contains DNA associated with proteins called histones and non-histone
chromosomal proteins.
Histones- small positively charged proteins mainly structural, that pack
DNA into chromatin fibers.
Nonhistone proteins- involved in DNA replication, transcription regulation
of gene activity
Chromosome: each individual linear DNA molecule with its associated
proteins;
23 pairs of chromosomes in humans (ranges from 1 to 50)
the same in all cells of an organism
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only visible during mitosis
in humans each chromosome contains about a meter of DNA
Chromatin: DNA and its associated proteins; the collection of
chromosomes at any phase of the cell cycle.
Gene: the information encoded in the DNA sequence necessary to make a
protein
Genome: all of the DNA, or hereditary material, in both functional and
nonfunctional sequences in an organism.
Cytoplasm: the region of the cell lying outside the nucleus.
Ribosomes: site of translation (protein synthesis); may be free in cytoplasm
or attached to endoplasmic reticulum.
Rough endoplasmic reticulum: ER with attached ribosomes; proteins
formed here go into ER to be further processed and/or distributed to vesicles for
transport to other parts of the cell.
Smooth ER: no ribosomes attached; functions include breaking down fats
and synthesis of lipids.
Golgi apparatus: direct membrane constituents to appropriate locations
within the cell; function in the formation of storage vesicles or secretory vesicles.
Lysosomes: organelles in animal cells that containing digestive enzymes.
Mitochondria: carry out the cell’s energy metabolism is carried out.
Mitochondria contain DNA. Function?
Peroxisomes: fatty acids and amino acids are degraded
Cytoskeleton: supportive network in the cytosol composed of three types of
fibers:
• Microtubles: fibers built of polymers of tubulin; responsible for cell
movement,
• Microfilaments, built of the protein actin
• intermediate filaments.
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The cytoskeleton gives the
cell strength and rigidity
control movement of
structures within the cell,
provide tracks along which
organelles move.
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[Lodish, 2000]
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Biomolecules:
Cells carry out a multitude of chemical transformations, which provide energy, and
the molecules needed to form its structure and coordinate its activities.
Small molecules: Water, inorganic ions, and a small array of small organic
molecules (e.g., sugars, vitamins, fatty acids), comprise 75-80% of living matter by
weight. These are imported into cells. Cells can also make or transform many
small molecules as part of the cells metabolic activity.
Macromolecules: proteins, polysaccharides, phospholipids and DNA. These are
made within cells.
• Carbohydrates: carbon hydrogen and oxygen (ratio 1:2:1); sugars
(monosaccharides) usually 3-7 carbons, straight chains or rings, found in
starches, cellulose, glycoproteins
• Lipids: diverse group of compounds that dissolve more readily in nonpolar
solvents than in water
Fatty acids
Saturated
Unsaturated
Polyunsaturated
Neutral lipids
Phospholipids
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• Proteins: the workhorses of the cell.
Proteins are the most abundant and functionally versatile of the cellular
macromolecules.
functions include structure, enzymes, mobility.
Proteins are formed from 20 different monomers, the amino acids. Linkages are
through peptide bonds.
Amino acids-subunits of proteins
• 20 different amino acids
• all but one have the same basic structure (central carbon, to which is
attached a carboxyl group and an amino group)
• the exception is proline (ring structure)
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[Lehninger, 1975]
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Structure defines function.
Primary structure: the linear sequence of amino acids -peptide bondcondensation
Secondary structure: the peptide bond is relatively rigid, relatively little rotation
so limited number of stable arrangements.
Tertiary structure- 3-dimensional arrangement; represents minimal energy state;
usually sensitive to temperature, pH, salt concentration, disulfide bonds help to
maintain conformation.
Quaternary structure- associations of multiple proteins. E.g., hemoglobin or
antibodies are made of 4 separate protein subunits.
Proteins can combine with other types of molecules (glycoproteins, lipoproteins).
Enzymes are proteins that carry out chemical reactions involving small molecules
by acting as catalysts. Tertiary structure brings reactants together, lowers free
energy barrier, and accelerates reactions by many orders of magnitude.
Inorganic ions may be required as cofactors for enzymatic activity (e.g., zinc,
copper, iron). “All (almost) enzymes are proteins, but not all proteins are
enzymes.”
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DNA: the cell’s master molecule.
Deoxyribonucleic acid or DNA is the macromolecule that garners the most public
attention.
As we explore the interaction of radiation with biological material, DNA will
assume central importance.
The structure of DNA was proposed in 1953 by James Watson and Francis Crick.
DNA consists of two long helical strands that are wound together in a double
helix.
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[Lodish, 2000]
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DNA structure
Nucleic acids (DNA and RNA) are linear polymers composed of monomers called
nucleotides.
All nucleotides have a common structure: a phosphate group is linked by a
phosphodiester bond to a pentose sugar (ribose in RNA and deoxyribose in
DNA)
The base components of the nucleic acids are heterocyclic ring structures called
purines or pyrimidines.
Purines: adenine, guanine
Pyrimidines: cytosine, thymine, uracil
The charge on the phosphate backbone gives nucleic acids a net negative charge.
Most nucleic acids in cells are associated with proteins.
Native DNA is a double helix of complementary antiparallel chains.
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The two sugar-phosphate backbones are on the outside. The bases are on the
inside connected by hydrogen bonds. Base-pair complementarity defines the
base bonding:
A-T
C-G
Also known as Watson-Crick base pairs.
Each strand of DNA is composed of 4 different monomers called nucleotides.
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Note that adenosine and guanosine are both purines, and thymidine and cytidine are pyrimidines.
A pairs with T, and G pairs with C.
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DNA is the storage form of genetic information.
DNA contains information about which proteins are to be made and when.
Proteins read the information in the DNA for use by the cell and translate it into a
transportable unit called messenger RNA (mRNA).
Other proteins read the mRNA sequence and assemble the amino acids into
proteins.
This is the central dogma of biology: the coded genetic information hard-wired
into DNA is transcribed into transportable cassettes, composed of mRNA, each
mRNA cassette contains the information for synthesis of a particular protein.
DNA → mRNA → protein
Gene expression is the process of translating the information in the DNA into
functional proteins.
Higher order structure: organizing DNA into chromosomes.
The total length of the DNA in a cell can be up to 100,000 times the cells diameter.
Packaging the DNA is crucial to the cell architecture and function. The longest
DNA molecules in human chromosomes (2-3 x 108 base pairs) are almost 10 cm
long!!
™ Eukaryotic nuclear DNA associates with histone proteins to form
chromatin.
Chromatin consists of an approximately equal mass of DNA and protein in a
highly compact structure known as chromatin. The general structure of chromatin
is similar in all eukaryotic cells.
The most abundant of the DNA-associated proteins are called histones. Histones
are a family of basic (positively charged) proteins present in all eukaryotic cell
nuclei. H1, H2A, H2B, H3 and H4
Histone sequences are highly conserved among species.
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™ Chromatin exists in extended and condensed forms.
Chromatin appearance depends on the conditions (primarily the salt concentration
or ionic strength) used in the solutions to isolate it from the cell.
At low salt concentration chromatin appears like “beads on a string” or in an
extended form.
The beadlike structures are called nucleosomes. Nucleosomes are composed of
DNA and histones, are about 10 nm in diameter and are the basic structural unit
of chromatin.
At high salt concentration, isolated chromatin assumes a more condensed fiberlike
form that is 30 nm in diameter.
Structure of nucleosomes;
• A protein core with DNA wound on its surface like thread on a spool.
• Nucleosomes contain146 base pairs wrapped slightly less than two turns
around the protein core.
• The DNA of nucleosomes is more protected from enzymes (and radiation?)
than the linker DNA between nucleosomes.
Structure of condensed chromatin:
• When extracted from cells in isotonic buffers, most chromatin appears as
fibers, 30 nm in diameter.
• Nucleosomes are thought to be packed into a spiral or solenoid
arrangement, with six nucleosomes per turn.
Modification of the histones (acetylation) can affect the binding and the assembly
into condensed chromatin.
Condensed chromatin structure may be dynamic, with sections partially unfolding
and then refolding.
Chromatin in regions not being transcribed exists in the condensed form, and
possibly higher order structures built from it.
Chromatin in regions actively being transcribed is thought to exist in the extended
“beads on a string” form.
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Eukaryotic chromosomes contain one linear DNA molecule.
It is difficult to isolate high molecular weight DNA without breaking it.
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[Lodish, 2000]
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Chromatin is further organized into chromosomes
Chromatin is further organized into large units hundreds of kilobases in length
called chromosomes.
Chromosome number, size and shape at metaphase (the karyotype) are species
specific.
In non-dividing cells the chromosomes are not visible.
During mitosis (and meiosis) the chromosomes condense and become visible.
Almost all cytogenetic work has been done on metaphase cells.
The condensation results from several orders of folding and coiling of the 30 nm
chromatin fibers.
At mitosis, cells have progressed through S phase and duplicated the DNA.
Metaphase chromosomes are composed of two sister chromatids.
Non-histone proteins provide a scaffold for long chromatin loops.
Megabase-long loops of 30 nm DNA fibers are thought to associate with the
flexible chomosome scaffold, yielding an extended form characteristic of
chromosomes during interphase.
Coiling of the scaffold into a helix and further packing of this helical structure
produces the highly condensed structure characteristic of metaphase
chromosomes.
Genes are located primarily within chromatin loops. Scaffold associated regions
serve as the attachment points of the loop to the protein scaffold.
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The Cell Cycle
• New cells arise when one cell divides or two cells (like sperm and egg) fuse.
• These events initiate a cell-replication program that is encoded in the DNA
and executed by proteins.
• A period of cell growth, and replication of DNA is then followed by cell
division.
• Cell growth and division is highly regulated in the body.
• Cancer occurs when a cell escapes from this regulation and growth is
unchecked.
Most eukaryotic cells live according to an internal clock, they proceed through a
series of phases called the cell cycle.
• G1: the gap between the end of mitosis and beginning of S phase
• S phase: DNA is duplicated: cells have a DNA content that progressively
increases from 2n to 4n.
• G2: time between the end of S and the beginning of mitosis
• M- mitosis: Cell division: the two daughter cells receive all of the genetic
information of the parent cell.
• G0: quiescent cells, not actively cycling.
G1 and G2 are periods of apparent inactivity between the major discernable events
in the cell cycle.
• Bacteria can replicate their single
chromosome and divide in about 20
minutes.
• Most mammalian cells have a cell
cycle time on the order of 10-12
hours.
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• Some cells (nerve cells, striated
muscle cells) do not divide at all.
They have temporarily exited from
the cell cycle and entered a
quiescent state called G0.
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Checkpoints
“Proofreading before division”
Progress through the cell cycle is regulated at key checkpoints along the way that
monitor the status of the cell.
Before entering mitosis, the integrity of the DNA is checked.
Exposure to radiation causes a block in G2.
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Mitosis is the process for partitioning the
genome equally during cell division.
The mitotic apparatus captures the
chromosomes and pulls them to the
opposite sides of the dividing cell.
• Prophase: the replicated
chromosomes (each containing two
identical chromatids) are condensed
and released to the cytoplasm when
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the nuclear membrane breaks down.
• Metaphase and anaphase: the
chromosomes are sorted and moved
to opposite ends of the cell.
• Telophase: marks the end of mitosis
as a membrane is reformed around
each set of chromosomes.
• Cytokinesis: division of the
cytoplasm and separation of the two
daughter cells.
[Lodish, 2000]
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What’s cancer?
Cancer-uncontrolled cell growth
• Tumor- mass formed by growing cancer cells
• Malignant- destructive, invasive, can metastasize
• Benign- slowly growing, does not invade surrounding tissues or metastasize,
usually encapsulated.
• Metastasis- spread of the tumor cells to other sites, where they establish
secondary areas of growth.
• Angiogenesis- both primary and secondary tumors require new blood
vessels to sustain growth.
E.g.,
• Carcinoma- solid tumor arising from epithelial tissue such as skin, colon,
lung, breast.
• Sarcoma- solid tumor arising from connective tissue such as bone.
Tumor
Malignant
Adenocarcinoma
Carcinoma
Glioma
Hepatoma
Leukemia
Lymphoma
Melanoma
Myeloma
Nephroblastoma
Neuroblastoma
Retinoblastoma
Sarcoma
Seminoma
Squamous
Usually benign
Adenoma
Chondroma
Fibroma
Osteoma
papilloma
Overview of Cell Biology
Tissue
Glandular
Epithelial
Glial cells in CNS
Liver
WBCs leukocytes
lymphocytes
Pigment cells
Plasma cells (bone marrow)
Kidney
Nerve cells
Eye (retina)
Connective tissue
Reproductive cells
Epidermal
Glandular
Cartilage
Fibroblasts
Bone
Surface epithelia
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Cancer cells can multiply in the absence of normally required growth factors and
are resistant to signals that normally program cell death.
Malignant cells show many differences from normal counterparts.
• Less well differentiated
• More rapid growth
• Loss of normal attachments to neighbors, loss of contact inhibition
• Growth factor independent cell growth
• Genetically unstable (phenotype and genotype can change with time)
• Disruptions to cytoskeleton
• Higher metabolic rate
• Changes in cytoplasmic ion concentrations
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Genes involved in cancer
Seven types of proteins participate in the control of cell growth.
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Oncogenes- cancer causing genes
Many oncogenes are altered forms of normal cellular genes called proto-oncogenes
Gain-of-function mutations convert proto-oncogenes into oncogenes.
Tumor suppressor genes- genes which code for cell cycle control proteins
• Loss-of-function mutations in tumor suppressor genes are oncogenic.
• Usually act recessively, both copies must be deleted or mutated to lose the
normal cell growth suppression effect.
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Multi-step progression of cancer
Development of cancer requires several mutations
Consistent with the observation of increased incidence as a function of age
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• Cancer: a clone from a single cell?
• Colorectal carcinoma well studied with biopsy material and genetic analysis
at all stages of development.
o Visible on endoscopy
o Biopsy material available
o Analyze genetic mutations
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Carcinogens- agents that can cause cancer
Viruses- can introduce oncogenes, or suppress genes that inhibit cell growth
• Retroviruses- RNA viruses, integrate into the genome of the infected cell as
a DNA copy and can carry oncogenes derived from cellular protooncogenes.
• DNA tumor viruses- contain oncogenes of purely viral origin
Chemicals- usually thought to act as carcinogens by causing DNA damage.
• Some of the most potent are alkylating agents that are capable of adding
organic groups to DNA.
• Others, (polycyclic hydrocarbons) become carcinogenic when they are
biochemically modified in cells, often as the cell attempts detoxification
• Phorbol esters- do not cause DNA damage, but promote growth (may only
work when DNA damage also occurs from another agent)
Radiation- also thought to act by causing DNA danage
• UV- is absorbed directly by DNA, causing base changes, in sunlight can
lead to skin cancer.
• Ionizing radiation is actually a relatively weak carcinogen.
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Apoptosis- now recognized that cancer is due not only to uncontrolled cell growth,
but also to loss of control in regulated cell growth.
In an adult, normal tissues are in homeostasis- no net increase in cell numbers
because of a balance between cell division and cell death due to apoptosis.
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Apoptosis vs. Necrosis
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